The invention relates to an engine-braking device for an internal-combustion engine which comprises a throttle valve arranged in the cylinder head of the engine to control a throttle port located between the cylinder and an outlet channel. A control piston for actuating the throttle valve is inserted in a control bore in a cylinder head and is subjected to compressed air during the braking operation. Bearings for rocker arms, arranged in a rocker-arm space in the cylinder head for actuating the gas exchange valves, are lubricated with lubricating oil from the lubricating-oil circuit of the internal-combustion engine. Lubricating oil dripping out of the bearings into the rocker-arm space is diverted through an oil return bore into the engine crankcase.
An engine-braking device of this general type is known from German Offenlegungsschrift No. 3,428,626. There a controlled piston is inserted in the control bore of the cylinder head and is actuated by means of compressed air. As a result of the high opening and closing speed of the control piston, relatively high frictional resistances arise between the piston and the bore wall. This structure provides a real danger in that the control piston can jam in the control bore.
The object on which the instant invention is based is to design the engine brake throttle valve in a constructionally simple way that its functioning and stability under load are improved substantially.
The object is achieved by having the oil return bore from the rocker arm space in the cylinder head have a connection to the control bore through an orifice.
As a result of the invention, lubrication of the control pistons is obtained in a simple way, without any additional construction outlay, by branching off some of the lubricating oil flowing out of the rocker-arm space into the crankcase and into the control bore. The lubrication achieves an increase in the stability under load, since the friction between the piston and control bore is reduced and a jamming of the piston in the control bore is largely prevented. Moreover, lubrication ensures an additional cooling effect on the control piston, as a result of which, the wear of a sealing ring arranged or the control piston decreases, thus likewise contributing to an improvement of the stability under load.
Connecting the oil return bore and the control bore also achieves a venting of the space located under the control piston, and therefore there is no need to make a special venting bore.
It is also advantageous if the control piston is stepped and has a sealing portion and guide portion. Here a connecting orifice is provided in a wall of the control bore adjacent to the guide portion. The control bore and the oil return bore are so arranged relative to one another, that the oil return bore intersects the control bore and with a common intersection line forming the edge of the connecting orifice.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.